The present invention relates to circuit breakers, and more particularly relates to over-current protection for circuit breakers.
Over current protection is provided by circuit breakers, fuses, and their associated switches, relays and other devices. Each type of equipment has variations in classes and ratings. Most types of over-current protection are designed to prevent damage to conductors and insulation from small or large excesses of current.
Circuit breakers, including molded case circuit breakers, protect against overheating of the conductor. The current path within a typical breaker is through a bimetallic strip; the resistance of the bimetal develops heat which causes the bimetal to bend until it moves far enough to unlatch the mechanism that allows the breaker to trip open. This is considered thermal tripping for overload currents.
The thermal action provides inverse time response. That is, a small overload takes a long time to heat the bimetal and trip the breaker. As the overload increases, the heating and tripping time is reduced. The larger the current, the less the tripping time, until the current reaches the setting of the magnetic trip. Magnetic response is instantaneous. Magnetic tripping is utilized for faster tripping at higher fault currents. Electronic circuit breakers may operate by sensing current level and trip when preset current is reached.
One known circuit breaker uses a temperature sensor to calibrate the bimetallic strip during the manufacturing of the product.
In an exemplary embodiment of the invention, a trip processing unit for a circuit breaker comprises a target for temperature measurement preferably located on the current path of the circuit breaker. The unit further includes a temperature sensor for reading the temperature of the target. A trip processor board within the circuit breaker receives temperature measurements from the temperature sensor. The temperature sensor triggers a trip signal in the trip processor board when a temperature of the target falls within a preselected critical range where further use of the circuit breaker could become detrimental to the conductors and insulation.